Sensing fibers may be used in different applications, for instance to control the functionality of fiber optic networks or to be installed like a sensing nerve system in a structure. These sensing fibers are primarily used for testing and can perform a similar function to a “strain gauge”.
The sensor fiber represents the physical condition of the main optical fiber line and by testing the operational condition of the sensing fiber, the environment of the main line can be determined.
Glass fibers are sensitive to temperature or pressure and tensile forces, which locally change the characteristics of light transmission and reflections in the fiber. Such sensing properties of the glass fibers make it possible to also incorporate sensor fibers in a long distance tubing, such as a pipeline, in order to detect and localize a deformation or a change in temperature.
The standard method for measuring reflections from a pulsed probe signal is Optical Time Domain Reflectometry (OTDR) and uses a combination of optoelectronic testing instruments, such as a laser source and a pulse generator, to inject a series of optical pulses into the sensing fiber end.
The optical pulses travel through the optic fiber and are continuously reflected back to the same fiber end where the pulses were initially injected. Other optoelectronic testing instruments (detector combined with an oscillator) receive and interpret the return signal from the same fiber end by measuring the back-scattered light. The back scattered light contains a large quantity of different information based on reflections from reference points along the fiber. The strength of the return pulses is measured and interpreted as a function of time, and can be plotted as a function of fiber length.
Since the first demonstration of distributed fiber sensing using OTDR, based on Rayleigh scattering, a variety of distributed fiber sensing systems has been extensively developed over last two decades, using different physical phenomena such as Raman and Brillouin scattering. Most distributed sensing techniques rely on spontaneous light back-scattering while the light propagates through a sensing fiber installed along a structure under monitoring. However, the efficiency of spontaneous light scattering in any OTDR system based on Rayleigh, Raman and Brillouin scattering is insufficient to achieve a high spatial resolution over a long measurement range.
The process of light scattering can be significantly enhanced based on optical parametric interactions between two optical waves such as stimulated Brillouin scattering, designated as Brillouin optical time-domain analysis (BOTDA). This type of sensing system has shown its potentiality to interrogate distributed temperature and/or deformation of structures over 50 km with 2 m spatial resolution. In principle, the spatial resolution is determined by duration of the Brillouin pump pulse. So, a higher spatial resolution (shorter than 2 m) can be achieved simply by reducing the pump pulse duration. However, the sensing system will suffer from significant spectral broadening of the Brillouin resonance, which is inversely proportional to the pulse shortening. Consequently, it will degrade sensing performances of BOTDA system in terms of measurement accuracy, increasing standard deviation.
Recently, dynamic Brillouin grating (DBG)-based distributed sensing (DS) system using polarization maintaining fibers has been experimentally demonstrated, resulting in a best spatial resolution of 5 mm, ever reported in time domain sensing systems. Unlike the typical BOTDA system, two distinct physical processes: generation of Brillouin grating and interrogation of grating properties are entirely separated in this type of sensing system. This way the trade-off relation between high spatial resolution and high measurement accuracy could be no longer correlated. However, two actual limitations are apparently present in this type of sensing system. First, the states of polarization (SOP) of optical waves can be maintained during longitudinal propagation over less than 1 km, strictly limiting a maximal achievable measurement range. Second, from a practical point of view, the complexity of the sensing system may act as an actual limitation for its implementation in real applications.
In addition to the previous cited sensing systems another category of sensors has been developed, namely Fiber Bragg Grating (FBG) sensors.
“Bragg gratings” are reference points along the sensing fibers. They usually consist of laser engraved patterns which have been imprinted along the whole optic fiber length at specific and pre-defined distances.
Because of these Bragg gratings, an undamaged testing fiber generates a predetermined and specific return signal to the OTDR testing tool. If the optic fiber sensor is subject to mechanical strain (due to thermal expansion, damage, break, heat, pressure, magnetic or electric field etc.), the OTDR receives a modified return signal and can determine the location of the damaged point if a Bragg grating is coincidentally present at this point.
Such FBG sensors are disclosed in the following two patents: U.S. Pat. Nos. 4,996,419 5,684,297. The fibers show a plurality of separate Fiber Bragg Gratings distant from each other. Each FBG has a relatively short length. As pointed out in U.S. Pat. No. 5,684,297, the spectral width of a short-length FBG is so large, so a long frequency scan of probe pulse is required. Furthermore, the power dissipation of the pulse is also significant since the pulse spectrum is much shorter than the FBG reflection spectrum.
The state-of-the-art FBG sensors show several disadvantages. They often require a pre-calibration of the relation between FBG peak frequency shift and detected optical power difference. Furthermore, changes of temperature and strain are not uniform along the fiber, which makes the initial FBG spectrum (measured as a reference) distorted. It means that the pre-calibration would turn to be ambiguous, which will definitely degrade the measurement accuracy.
There is therefore a need to improve existing FBG sensors.